1. Field of the Invention
The present invention relates to an optical transparent member having an antireflection performance and an optical system using the same, and more particularly to an optical transparent member suitable for obtaining a high antireflection performance from a visible region to a near infrared region over a long period of time, and an optical system using the same.
Particularly, the optical transparent member of the present invention can adapt to a transparent base having any refractive index, shows an excellent antireflection effect to visible light, and has a long-term weathering resistance, and therefore it can be used for optical members of various kinds of displays of word processors, computers, televisions, plasma display panels and the like; polarizing plates of liquid crystal apparatuses; sunglass lenses, graduated eyeglass lenses, finder lenses for cameras, prisms, fly-eye lenses, toric lenses, and various kinds of optical filters and sensors and the like consisting of various kinds of optical glass materials and transparent plastics; and further, optical members of various kinds of optical lenses of image pickup optical systems using those optical members, observation optical systems such as binoculars, projection optical systems for use in liquid crystal projectors, scan optical systems for use in laser printers and the like, covers of various kinds of instruments, and window glasses of automobiles, electric trains and the like.
2. Related Background Art
It is known that an antireflection structure using a fine periodic structure having a wavelength of the visible light region or a shorter wavelength forms a fine periodic structure having an appropriate pitch and height, and thereby shows an excellent antireflection performance in a wide wavelength region. As a method for forming a finely periodic structure, coating of a film in which fine particles having a particle diameter equal to or less than the wavelength are dispersed (Japanese Patent No. 03135944) or the like is known.
It is known that a method of forming a fine periodic structure by formation of a pattern by a fine processing apparatus (electron beam lithography apparatus, laser interference light exposure apparatus, semiconductor light exposure apparatus, etching apparatus, etc.) allows a pitch and a height to be controlled, and makes it possible to form a fine periodic structure having an excellent antireflection property (Japanese Patent Application Laid-Open No. S50-70040).
As methods other than the methods described above, methods of growing boehmite that is an oxide hydroxide of aluminum on a base to obtain an antireflection effect are known. In these methods, a layer of aluminum (alumina) formed by the vacuum film formation process (Japanese Patent Publication No. S61-48124) or the liquid phase process (sol-gel process) (Japanese Patent Application Laid-Open No. H9-202649) is subjected to a water vapor treatment or a hot water dipping treatment to form a surface layer into boehmite to form a fine periodic structure, and thereby an antireflection film is obtained.
However, in a technique using fine particles, it is difficult to control the pitch and height of the fine periodic structure, and if a height for obtaining a sufficient antireflection effect is to be obtained, the pitch increases to cause scattering, and conversely, the light transmittance decreases.
The method of forming fine patterns by the fine processing apparatus has a disadvantage that not only such a method of forming a pattern requires very large-scale equipment, thus requiring a very high capital expenditure, but also although the method is suitable for formation of a pattern on a flat surface, it is very difficult to form a pattern on a complicated shape such as a curved surface. In addition, the method is unsuitable for application to general-purpose optical elements because throughput is low and processing on a large area is difficult.
The method of growing boehmite on a base is convenient and has a high productivity, but alumina and boehmite are amphoteric compounds and thus are easily decomposed by acids and alkalis. Consequently, when alkali ions and the like of the base migrate to the surface and the surface is brought into an alkali atmosphere due to an exchange reaction with water in air, maintenance of a shape of irregularities becomes difficult due to decomposition of the surface and thus the performance is degraded. For a base having a refractive index significantly different from that of alumina, a difference in refractive index at an interface between the base and alumina is so large that the antireflection performance is not sufficiently exhibited.